This invention relates to methods and devices for detecting constituents of breathing air in general and, in particular, to a new and useful method for determining the most advantageous measuring instant for testing alcohol content in exhaled breath and to an apparatus for carrying out the aforesaid method.
In accordance with one known concept of the mechanism of alcohol transfer from the human body to the breath, a reliable breath alcohol test can only be taken after a volume corresponding at least to the dead space of the person whose breath is to be checked has already exhaled the dead space being that part of the respiratory tract where no gas exchange between blood and breathing air occurs. Moreover, it is necessary to wait long enough for the socalled "deep lung air" (alveolar air) to be present in the exhaled air and for the alcohol concentration consequently having assumed a saturation value.
More recent test results partly contradict this concept. According to such tests, the alcohol concentration in the exhaled air starts rising spontaneously from the start of exhalation. A conclusion can be drawn from these tests that the alcohol concentration in exhaled breath is not only a result of the gas exchange with the blood effected in the lungs but it is quite significantly determined by the alcohol content of other body fluids present in the respiratory passage, i.e., the said dead space.
One known arrangement for the determination of alcohol concentration measures the alcohol in exhaled breath at a point in time fixed by a timing device. This point in time is determined by the lapse of a settable time interval beginning within the exhalation time span. During this time interval, the breath flow rate must not drop below a fixed minimum flow rate, and the flow must always be in the exhalation direction only. If these two conditions are not met, an error detector will signal the invalidity of the test. The set time interval is to assure that the test person has already exhaled the air from his oral cavity and windpipe at the measuring instant, and that the test instrument then measures the alcohol concentration of the breath from the alveoli of the lungs.
The lapse of the set time interval is determined by the time when a minimum breath volume of preferably at least 80% of the entire breath volume has been exhaled. An integrator can time-integrate the breath flow rate during inhalation and exhalation and determine therefrom the lapse of the time interval by the minimum breath volume. This embodiment is supposed to be unaffected by the physical build of the test person, but the method is not error-proof in cases where the test person is uncooperative. A much too small breath capacity can be feigned by intentionally shallow inhalation. The minimum breath volume then establishing itself automatically, e.g., at 80% of the total breath volume, can then stem practically, from the oral cavity and the throat area only for the test. The alveolar air, which is decisive for an accurate test result, is then not picked up fully (See German Offenlengungschrift No. 24 28 352).
In another known test method and in the breath alcohol measuring instrument designed according to this method, both the CO.sub.2 content and the alcohol content in the exhaled breath are measured. Starting from the idea that the CO.sub.2 content is a measure for the O.sub.2 -exchange in the lung, a high CO.sub.2 content must point to breath from the lung. For the test, the test instrument first measured continuously the CO.sub.2 content in the exhaled air in order to switch on the alcohol measuring section after the attainment of the predetermined threshold value of 4.5% CO.sub.2 in the embodiment example, in order to then measure the breath alcohol content.
One inaccuracy of this method is inherent in the individual CO.sub.2 values which are subject to wide variations. A generally valid threshold value can, therefore, not be fixed. One test person will not reach the threshold at all, while in others, no air from the lungs may be present yet, although the threshold has been exceeded. Furthermore, an instrument to measure the concentrations of two different gases is rather complicated and sensitive (See U.S. Pat. No. 3,830,630).
Another known method and the associated arrangement starts with the premise that the actual alcohol concentration in the breath is detected only, if that portion of the exhaled air which could find its equilibrium with the blood alcohol concentration in the alveoli of the lungs is examined for its alcohol content. Therefore, the reciprocating air from the mouth and throat area and the mixed air must be separated from the alveolar air for test purposes.
The method, and also the associated arrangement solve this problem by means of an infrared measuring instrument which continuously measures the momentary alcohol concentration while the sample is being taken. A threshold comparator determines the time variation of the measured values, which represents the measure of the rate with which the alcohol concentration increases.
A measured value is transmitted for display only if the rate of increase falls below a given threshold. This first condition results from the fact that the percentage of reciprocating air from the mouth and throat area becomes smaller and smaller as the rate of increase drops and the alveolar air only is still found in the measuring chamber of the arrangement when the threshold has fallen below. As another condition for the transmission of the measured value, the flow velocity of the exhaled air, as determined by a flow meter, must have been above a given value during a given time span up to the transmission of the measured value. This second condition makes certain that the test method progresses as intended. The alcohol concentration is measured by a fast responding infrared measuring instrument inserted in the breath flow. A disadvantage of this arrangement is that, because of the high resolution of the measured values as required for the determination of the rate of increase, an expensive infrared measuring instrument is needed. The reliable detection of the percentage of alveolar air is not feasible with simple, inexpensive, but slow alcohol measuring instruments.
The state of the art can generally be divided into the following two categories:
(a) Exhalation of a minimum volume as product flow x time, as intergral over the flow or volumetric; and PA1 (b) correlation with the time curve of the gas parameters varying during exhalation, such as temperature, alcohol concentration, or CO.sub.2 concentration.
The disadvantage in category (a) is that the minimum volume is not fixed individually for each test person, and new tests are not taken into account which have proven that the level of alcohol concentration reached is not only a function of the exhaled volume, but just as much a function of time. The alcohol figures differ when one and the same volume of air is exhaled intentionally fast or intentionally slow. If the test person holds his breath temporarily before exhaling, a high alcohol figure already results after exhaling a relatively small volume. Another characteristic of category (a) is that the direction of the flow must also be monitored so that it is recognized whether the person inhales fresh air.
A disadvantage of category (b) is that costly equipment, such as an alcohol sensor of high time resolution, or an additional CO.sub.2 sensor must be used, or that the correlation as over the temperature cannot be proven reliably.